Estimating impact of food choices on life expectancy: A modeling study [pdf] (ncbi.nlm.nih.gov)

Interesting paper that suggests moving from a standard American diet to an Optimal Diet at the age of 60 could still increase your life expectancy by as much as 10 years (95% will see an increase of between 6.8 years - 10 years life expectancy). If you started at 20 you could see an extra ~15 years in your life.

There's a little info on what an "Optimal Diet" supposedly is here https://en.wikipedia.org/wiki/Life_extension#Optimal_diet